OF HARTING. 3OQ 



position as the tongue in the mammalia, and the roof 

 of the mouth is supplied with a horny plate, against 

 which the open end of the ribbon can work backwards 

 and forwards, so as to rasp and triturate the food 

 between them. The tubular portion of this lingual 

 ribbon is contained in a cavity behind the mouth, and 

 as the teeth in use become worn or broken down, it 

 is conjectured that they are absorbed, and a fresh set 

 from the reserve in the tube is pushed forward to take 

 their place. 



The body of every mollusc with which we are 

 concerned, except those of the slug family, is con- 

 tained in a membranous sac called the mantle, which 

 not only serves as a model on which the shell is 

 moulded, but is liberally provided at the edges of its 

 open end with the glands that secrete the shelly 

 matter. To this set of glands alone are due the 

 coloured bands and other markings in the shells, as 

 may be seen in the case of a fractured specimen that 

 has been repaired by the tenant, in this the new 

 matter thrown out by the mantle under the fracture is 

 always colourless. Respiration in the animals of this 

 class is carried on by means of a rudimentary lung in 

 each member of the terrestrial division, the aquatic 

 species, with some exceptions, being provided with 

 gills. In the air-breathing molluscs the air is admitted 

 into the pulmonary cavity, not by the mouth, but by 

 an aperture which may easily be seen in slugs at the 

 edge of the convexity on the back formed by the 

 mantle, and in snails just within the mouth of the 

 shell, in both cases on the right side. We are all 

 tolerably familiar with the fact that one essential 

 character of the vertebrated animals is their possession 

 of a brain and spinal cord, from which proceed those 

 numerous 



" Tracts, along which the mysterious will 

 Is conveyed by a process on which fancy lingers 

 With awe, from the brain to the tips of the fingers" 



